1. Field of the Invention
The present invention relates to optical communications systems and, more particularly, to lasing semiconductor optical amplifiers.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of data over a telephone company""s trunk lines, the transmission of images or video over the Internet, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth capacity will only increase.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the OC protocol have been developed for he transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal.
Optical amplifiers, which boost the power of the optical signal propagating through the optical fiber, are an important component in such fiber communications systems. For example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers can be used to boost the received optical signal to the proper power range for the receiver. As another example, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers may be used to compensate for these attenuations. Conventional optical amplifiers, however, suffer from various drawbacks.
Fiber amplifiers are one type of conventional optical amplifier. They include a length of fiber which has been doped to form an active gain medium. Ions of rare-earth metals, such as erbium, are typically used as the dopant. The doped fiber is typically pumped by an optical pump at a wavelength which is preferentially absorbed by the ions but different from the wavelength of the optical signal to be amplified. The pumping results in a population inversion of electronic carriers in the active medium. Then, as the optical signal propagates through the doped fiber, it is amplified due to stimulated emission.
One drawback of fiber amplifiers is that they typically can only operate over a narrow wavelength range when multiple fiber amplifiers are cascaded. This is especially problematic if the optical signal to be amplified covers a wide range of wavelengths, as would be the case if the entire bandwidth of the optical fiber is to be efficiently utilized. Another disadvantage of fiber amplifiers is their transient response to channel drop-out in wavelength division multiplexing systems. Further problems with fiber amplifiers include their relatively large size, slow switching speed, power inefficiency, difficulties in mass producing them, and their high cost which makes them prohibitively expensive for many applications.
Non-lasing semiconductor optical amplifiers (SOAs) are an alternative to fiber amplifiers. Non-lasing semiconductor optical amplifiers are typically based on a semiconductor laser-like structure which is operated below the lasing threshold. Typically, an electrical current pumps the active region of the amplifier, resulting in an increased carrier population. The optical signal then experiences gain as it propagates through the active region due to stimulated emission.
One problem with non-lasing semiconductor optical amplifiers is that the gain depends on the amplitude of the optical signal. For example, a strong optical signal will be amplified less than a weak signal and strong portions of the optical signal will be amplified less than weak portions. This results in distortion of the optical signal and possibly also crosstalk between different optical signals propagating simultaneously through the system. This problem is the result of gain saturation, in which there are insufficient carriers in the conduction band to provide the full amount of gain to higher power signals.
Lasing semiconductor optical amplifiers can overcome the problem of gain saturation. These amplifiers are also based on a semiconductor active region. However, the active region is pumped above the lasing threshold. The gain is then clamped due to the lasing action and is fairly constant until the amplifier reaches its power limit.
However, lasing semiconductor optical amplifiers also suffer from inherent drawbacks. For example, there is an inherent tradeoff between noise performance and power output. If the carrier density at the lasing threshold is high, the amplifier will have good noise performance but will have a low saturable power thus limiting its power output. On the other hand, an amplifier with a low carrier density at the lasing threshold will be capable of large power output but suffer from poor noise performance. This inherent tradeoff makes it difficult for a lasing semiconductor optical amplifier to attain both a low noise and a high power output.
Thus, there is a need for an optical amplifier which does not suffer from gain saturation and is also capable of both low noise and high power output.
In accordance with the present invention, a multi-stage lasing semiconductor optical amplifier (SOA) device for amplifying an optical signal includes at least two SOA stages coupled in series. Each SOA stage includes a semiconductor gain medium, a laser cavity including the semiconductor gain medium, and a pump input to the semiconductor gain medium. The semiconductor gain medium has an amplifying path along which the optical signal to be amplified propagates. The pump input receives a pump which pumps the semiconductor gain medium above a lasing threshold for the laser cavity. The onset of lasing clamps a gain of the semiconductor gain medium to a gain value which is substantially independent of the amplitude of the optical signal and the optical signal is amplified as it propagates through the semiconductor gain medium. The SOA stages are characterized by a design parameter which varies from stage to stage. The design parameter preferably includes a noise figure and a saturable power for each SOA stage, with both parameters increasing from stage to stage.
In a preferred embodiment, the optical signal propagates along the semiconductor gain medium, which forms part of a waveguide. The laser cavity in each SOA stage includes a first and a second Bragg reflector disposed to form a laser cavity oriented vertically with respect to the amplifying path. The reflectivity of the Bragg reflectors increases from stage to stage and, accordingly, the noise figure and the saturable power also increase from stage to stage. The semiconductor gain medium is pumped by a pump current injected via an electrical contact and these preferably are multiple electrical contacts for each SOA stage.
In further accordance with the invention, a method for amplifying an optical signal utilizes a multi-stage lasing semiconductor optical amplifier (SOA) device comprising at least two SOA stages, each SOA stage including a semiconductor gain medium and a laser cavity including the semiconductor gain medium. The method includes the following steps. The optical signal to be amplified is received. For each SOA stage, the optical signal propagates along an amplifying path in the semiconductor gain medium. The semiconductor gain medium is pumped above a lasing threshold for the laser cavity, whereby a gain of the semiconductor gain medium is clamped to a gain value which is substantially independent of the amplitude of the optical signal. The optical signal is amplified as it propagates along the amplifying path. The amplification is responsive to the gain value of the semiconductor gain medium and to a value for a design parameter for each SOA stage. Furthermore, the value of the design parameter varies from stage to stage.
In another aspect of the invention, a varying lasing SOA device for amplifying an optical signal includes a semiconductor gain medium, a laser cavity, and a pump input. The semiconductor gain medium has an amplifying path and the optical signal is amplified as it propagates along the amplifying path. The laser cavity includes the semiconductor gain medium. The laser cavity is off-axis with respect to the amplifying path and varies along the amplifying path. The pump input is coupled to the semiconductor gain medium. A pump received via the pump input pumps the semiconductor gain medium above a lasing threshold for the laser cavity, whereby a gain of the semiconductor gain medium is clamped to a gain value which is substantially independent of the amplitude of the optical signal. In a preferred embodiment, the noise figure and saturable power for the laser cavity increases monotonically along the amplifying path.
The present invention is particularly advantageous because the use of multiple SOA stages characterized by different design parameters and/or the variation of the laser cavity along the amplifying path results in increased flexibility in the design of the lasing SOA device. For example, by allowing the noise figure and saturable power to vary from stage to stage, better noise performance and higher saturable power for the lasing SOA device can be achieved.